CHEMICAL PREPARATION OF THE BINARY
IN THE CaO-Al2O3 SYSTEM BY SELF-PROPAGATING COMBUSTION SYNTHESIS
Department of Metallurgical and Materials Engineering,
Middle East Technical University,
Ankara 06531, Turkey. (1993-1999)
* A. C. TAS, “Chemical Preparation of the Binary Compounds of CaO-Al2O3 System by Self-propagating Combustion Synthesis,” Journal of The American Ceramic Society, 81, 2853-2863 (1998). (------> download pdf: cao-alo.pdf)
* O. Uysal and A.
C. TAS, ”Chemical Preparation of the Binary Compounds of CaO-Al2O3 System by
Combustion Synthesis,” in Innovative Processing and Synthesis of Ceramics,
Glasses, and Composites, Ceramic Transactions, Vol. 85 (ISBN 1-57498-030-0),
pp. 39-54. Eds., Narottam P. Bansal, Kathryn V.
Logan, and J.P. Singh, The American Ceramic Society,
* O. Uysal and A. C. TAS, "Chemical Preparation of Some of the Binary Compounds of CaO-Al2O3 System by Self-Propagating Combustion Synthesis (SPCS)," III. Ceramics Congress, Proceedings Book, Vol. 2, pp. 428-439, October 1996, Istanbul, Turkey.
* A. C. TAS, "
* A. C. TAS, “Low-temperature Chemical
Synthesis of Ceramic Powders of Calcium Aluminate Binary Compounds”
Patent Application, Date:
* O. Uysal and A.
C. TAS, “Chemical Preparation of the Binary Compounds of CaO-Al2O3 System by
Self-Propagating Combustion Synthesis,” 99th Annual Meeting of the American
The binary compounds (C3A: Ca3Al2O6, C12A7: Ca12Al14O33, CA: CaAl2O4, CA2: CaAl4O7 and CA6: CaAl12O19) in the CaO-Al2O3 system have been synthesized as high compound purity ceramic powders by using the Self-Propagating Combustion Synthesis (SPCS) method. Materials characterization of the above-mentioned phases were performed by powder XRD, FTIR, SEM and EDXS. The structural characterization of the C12A7 phase has been performed by Rietveld analysis on the powdered XRD samples. It has hereby been shown that by using this synthesis procedure it would be possible to manufacture high purity ceramic powders of CA, CA2, and C12A7 at 850°C, of C3A at 1050°C, and of CA6 at 1200°C in a dry air atmosphere.
The binary compounds of the CaO-Al2O3 system hold a significant place in a wide spectrum of applications in metallurgical slags, ceramic materials, and cement technology. The superior refractory properties of these binary line compounds, which lie between the 2900°C-melting CaO and 2050°C-melting Al2O3 terminal members, make them progressively attractive, in recent years, in the cement manufacturing technology. Cements containing these binary compounds are especially used in casting, trowelling and gunning applications. The pure, alkali-free binary compounds of the CaO-Al2O3 system are also being considered for replacing the alkali-containing chemical additives used in cement technology.
The chemical and thermodynamic properties of the CaO-Al2O3 system, as well as of the above-mentioned binary line compounds, were recently compiled and assessed by Hallstedt , and by Eriksson and Pelton . Ca3Al2O6 (C3A) is known to melt incongruently  at 1544°C, by transforming into a mixture of a liquid phase and CaO. C3A, also known as tri-calcium aluminate, is mainly used in Portland cement compositions rather than high-alumina cements. The preparation of this compound by conventional methods (mixing and milling of CaO and Al2O3 in stoichiometric amounts followed by solid-state reactive firing in kilns) has always been troublesome, and the final product of conventional syntheses almost always yields the other calcium aluminate compounds, together with some unreacted CaO and/or Al2O3 as impurity phases. CaAl2O4 (CA) melts congruently at about 1600°C [4-7] and, when prepared by conventional methods, the final product of solid-state reactive firing contains the impurity phases of CaO, CA2 and C12A7 below 1300°C. Re-heating of this intermediate phase mixture at about 1450°C, after an homogenization milling, would then produce a single-phase CA powder body. On the other hand, high-purity and single-phase CA powders were also reported by Gulgun et al.  to be chemically prepared at temperatures below 900°C by using a Pechini-type  synthesis process. CA compositions were prepared  by the sol-gel technique by using the starting materials of Al-sec-butoxide and Ca-nitrate. CaAl4O7 (CA2) melts congruently at 1745°C  (or 1775°C ) and is also known as calcium di-aluminate. CA2 is preferred in use to a large extent among all the high-alumina cements, and is utilized as a high commercial value chemical substance, especially in casting, trowelling and gunning applications. The typical temperature of synthesis of this calcium aluminate compound, in conventional practices, is over the temperature range of 1350 to 1450°C. CaAl12O19 (CA6) melts incongruently [1, 12] at 1885°C, by transforming into a mixture of a liquid phase and ?-Al2O3. CA6 does not take its place among high-alumina cements since it is stable against water, and since it does not get hydrated, in contrast to other (i.e., CA, CA2, C3A and C12A7) binary calcium aluminates. When CA6 is blended with other calcium aluminates, it is found to cause a decrease in the mechanical strength of the cement .
Cinibulk and Hay  studied the evolution of CA6 phase from alumina sols containing calcium acetate. They reported that after calcining the gelled sols at 1200°C, CA6 was the major phase together with the significant presence of alpha-Al2O3 and CA2. Following the air calcination at 1400°C, a nearly single-phase powder of CA6 was obtained  still displaying the traces of alpha-Al2O3 and CA2. CA6 sols were also used  to coat single-crystal yttrium-aluminum-garnet (YAG) fibers and alumina plates with the hibonite (CA6) phase, and then to study the behavior of the textured fiber-matrix interphases.
An and Chan  have studied the microstructural and mechanical properties of Al2O3:CA6 ceramic composites manufactured by the reactive-sintering of alumina, and calcia or calcium carbonate mixtures heated at the peak temperature of 1650°C for 2 hours. It was noted in this study that the enhanced toughening behavior observed in the samples was mainly due to the crack bridging mechanism provided by the in-situ formation of the CA6 platelets. The control of sintered grain morphology  and the mechanical properties  of the CA6-containing ceramic composites have also been studied.
The refractory properties of the CA6 phase, and its application in high-temperature calcium aluminate cements have previously been discussed by Kopanda and MacZura .
C12A7 (Ca12Al14O33) has previously been shown not to be stable in the anhydrous CaO-Al2O3 system [1-2, 6]. The earlier determinations [4, 18] of the phase diagram have identified four intermediary phases: C3A, C5A3, CA, and C3A5. The phases C5A3 and C3A5 were later assigned the formulas C12A7 and CA2, respectively [1-2, 19]. Nurse et al. [6, 20] determined the phase diagram in a moisture-free atmosphere and concluded that C12A7 is not stable under strictly anhydrous conditions. The formula Ca12Al14O32(OH)2 has been proposed for this phase . The structural ambiguity is still believed to persist over this compound. The compound C12A7 has previously been synthesized [22-23] by the solid- state reactive firing of reagent-grade starting materials like CaCO3, CaO, or Al2O3 mixed in appropriate amounts. The formation of the C12A7 phase necessitated the attainment of temperatures in excess of 1400°C with equilibration times higher than 24 hours. It has been reported by Roy et al.  that the final product may contain up to 1.30 to 1.40 wt% H2O (corresponding to the composition C12A7H) after heating to about 1100°C in air of normal humidity. This water was claimed to be absorbed reversibly and without any major structural change; therefore Roy et al.  termed C12A7 a zeolitic phase.
Several binary compounds, including the C12A7 phase, in the CaO-Al2O3 system have been synthesized by Kuznetsova et al.  via the preparation of mixed Al-Ca-hydroxides in aqueous solutions. This report claimed the reduction of the synthesis temperature of C12A7 to less than 1200°C, but the chemical precipitation process employed was not able to produce a single-phase substance, and the solid product of the subsequent air calcination was also found to contain other calcium aluminate phases amounting to about 5% in the calcined body.
The structure of the C12A7 phase was first studied by Eitel et al.  and they reported a cubic structure with the lattice parameter, a=11.95 Å, and a possible space group of Td6. They claimed that this structure is built up by the 12:7 site ratio, and a three-dimensional network of AlO4 tetrahedra constitutes the back bone of the structure while all oxygen atoms belong to every two such tetrahedra. Glasser et al.  did later determine the space group of the C12A7 unit cell to be Td6 or I-43d with a lattice parameter of 11.98 Å. Only 64 of the 66 oxygen atoms in the unit cell could be placed in this space group; the remaining two were assumed to be distributed statistically. The structure of the fluoride analogue (i.e., 11CaO.7Al2O3.CaF2) of cubic C12A7 was refined by Williams  and the structure was confirmed to belong to the space group I-43d with a=11.970 Å, Z=2.
It has been reported  that fine particle oxide ceramics could be produced using exothermic redox reactions between an oxidizer (metal nitrates) and a fuel (amides, hydrazides, etc.). This concept was first demonstrated by Kingsley et al.  on the rapid synthesis of fine particle alpha-Al2O3 and related oxides, such as metal aluminates, rare-earth orthoaluminates, and Ce3+ or Cr3+-doped aluminum oxides. The process involved the combustion of corresponding metal nitrate plus either urea or carbohydrazide mixtures at 500°C or 250°C, respectively, under normal atmospheric pressure . The process yields foamy, voluminous and fine oxide powders in less than 5 minutes. The combustion being instantaneous and energy saving has attracted much interest and been successfully utilized in the synthesis of LaCrO3 , Ba2YCu4O8  and Y-Ba-Cu-O phases . Recently, combustion methods using glycine as the fuel , and urea as the fuel  have been reported for the successful synthesis of Ca-doped LaCrO3 and LaAlO3 powders, respectively. A similar combustion technique was also demonstrated for the successful synthesis of YAG:Cr and Y2O3:Eu , and of YAG:Nd and YIG:Nd  phosphor powders using both of the above-mentioned fuels.
In the present work, the
experimental conditions and parameters of the preparation of the ceramic
powders of the binary compounds of the CaO-Al2O3 system have been studied and presented
by the powder route of self-propagating combustion synthesis.
The binary compounds (C3A, C12A7, CA, CA2, and CA6) of the CaO-Al2O3 system were, for the first time, prepared by the self-propagating combustion synthesis. Significant decreases (C3A: 1050°C, C12A7: 800°C, CA: 850°C, CA2: 900°C, and CA6: 1200°C) in the synthesis temperatures (together with improved compound purities attained in the final powder bodies) of these compounds have been achieved as compared to the conventional methods and practices of solid-state reactive firing of the starting oxides (i.e., CaO and Al2O3), which require the operation temperatures in the range of 1400 to 1550°C for prolonged times in kiln-type furnaces.
Urea used (as a fuel and/or oxidizer) in the combustion synthesis runs was, later, separately replaced (in the initial aqueous solutions) in a series of experiments with carbohydrazide (CH6N4O) and glycine (C2H5NO2). It was noted that the samples of C3A, C12A7, CA, CA2, and CA6 prepared with the proprietary amounts of either carbohydrazide or glycine all yielded single-phase, pure (deduced only by XRD and EDXS analysis) binary calcium aluminates, followed by isothermal heatings (for 48 to 72 hours) at 1050, 800, 850, 900, and 1200°C, respectively.
The SEM micrographs of all the samples showed the presence of micron-range, irregularly shaped particles after calcination at each temperature. The FT-IR spectra of the combustion synthesized calcium aluminate precursor powders exhibited the typical nitrate (NO3) vibrations over the range of 1250 to 1650 cm-1. The nitrate peaks in the FT-IR plots disappeared with the calcination temperature increasing beyond 1000°C.
Once debated and heavily questioned (as its existence) phase, Ca12Al14O33 (C12A7), of the CaO-Al2O3 binary system has been synthesized, for the first time, by the self-propagating combustion synthesis technique, and a significant reduction in its synthesis temperature has been achieved with respect to conventional routes of solid-state reactive firing practices. The structural ambiguity on this compound has also been resolved, and the structural parameters and the unit cell contents of Ca12Al14O33 are hereby refined and presented. This phase (together with other binary calcium aluminates) is also expected to find increasing use in the field of alkali-free, synthetic chemical additives in cement compositions.
Drawn by Prof. Dr. A. Cuneyt Tas (1996)
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